Fix invalid start date cache

Fixed a bug, that crashed the website, because of a cached start date.

I, the author of this PR checked the following requirements for good software quality:

[x] The code is properly formatted (I ran the formatter)
[x] The code is written with our software quality standards (I ran the linter)
[x] The code is written using our code style
[ ] ~~Extensive in source documentation has been added~~
[ ] ~~Unit and/or integration tests have been added~~
[x] All texts have been internationalized with at least the following languages:
- [x] English
- [x] German
[x] I tried addressing all new accessibility problems displayed in the console and documented if they can't be fixed
[ ] ~~I attached performance measurements to prevent performance degradation~~
[x] I added the changes to the next release section of the changelog

I, the reviewer checked the following things:

[x] I ran the software once and tried all new and related functionality to this PR
[x] I looked at all new and changed lines of code and commented on possible problems
[x] I read the added documentation and checked if it is understandable and clear
[x] I checked the added tests for completeness
[x] I checked the internationalized strings for spelling errors
[x] I checked the performance metrics for problems or unexplained degradation
[x] I checked that the changes are noted in the changelog

DLR-SC / ESID